In apparatus for processing a sheet of yarns disposed side by side, for example a warping machine, it sometimes happens that the tension on one or more ends of yarn becomes excessive. Unless this excessive tension is detected and corrected quickly, the elastic limit of the yarn may be reached with the result that the yarn breaks. Moreover, excessive tightness may cause a change in the molecular characteristics of the yarn material, causing problems in subsequent operations such as draw-warping dyeing, weaving and knitting. It is especially important that a tight end detecting device react very quickly, usually within a few milliseconds, so that it is able to produce a signal to stop the operation of the machine before breakage occurs.
There is known a tight end detector in which a sensing finger element traverses the width of the yarn sheet, "feeling" the tension of each end individually as it passes by. This method is usually too slow to detect tight ends before breakage occurs. Moreover, the device is very expensive.